1. Field of the Invention
The present invention relates to novel platinum catalysts which can be activated by ultraviolet and/or visible radiation, their preparation, their use in crosslinkable silicone compositions and also the silicone elastomers produced therefrom by irradiation.
2. Description of the Related Art
In general, the crosslinking process in addition-crosslinking silicone compositions occurs via a hydrosilylation reaction in which platinum or another metal from the platinum group is usually used as catalyst. In the catalytic reaction, aliphatically unsaturated groups are reacted with Si-bonded hydrogen in order to convert the addition-crosslinkable silicone composition into the elastomeric state by formation of a network.
According to the prior art, the activation of the catalysts used is normally carried out thermally, i.e. the addition-crosslinkable silicone composition consequently has to be heated for the crosslinking process. According to the prior art, in this process, the silicone composition frequently has to be applied to a substrate, as is the case in, for example, coating processes, selected casting, molding and coextrusion or other shaping processes. The actual vulcanization process is in this case effected by means of a heating process for which costly and energy-consuming plants frequently have to be operated.
In comparison, a sometimes considerable cost saving is associated in many applications with the use of mixtures which can be crosslinked by ultraviolet and/or visible radiation. Consequently, an energy saving and process cost saving and thus a corresponding increase in productivity can be achieved. In addition, crosslinking by means of ultraviolet and/or visible radiation often allows continuous manufacture which brings further productivity advantages compared to a discontinuous batch process. A further advantage arises from the fact that, particularly in the case of multicomponent parts such as hard-soft composites which contain an elastomeric material together with, for example, a thermoplastic as the composite partner, the omission of a temperature-intensive manufacturing step prevents thermal distortion of the part.
The technical literature describes many platinum complexes which are suitable for initiating a hydrosilylation reaction by means of radiation. All platinum catalysts described can be activated by light and are also capable of crosslinking silicone compositions even after the light source is switched off. This process is known as dark reaction to those skilled in the art.
EP 0 122 008 B1 describes UV-crosslinkable silicone compositions containing a (η-diolefin)(σ-aryl)platinum complex as photosensitive catalyst. A high catalytic activity is indicated as advantageous. However, this class of catalyst nevertheless has a moderate dispersibility in the silicone matrix. In addition, the light-induced decomposition of the platinum catalyst requires the use of very short-wavelength UV-C radiation which inevitably leads to high ozone pollution in the direct vicinity of a production line.
EP 0 561 919 B1 describes a process for radiation-crosslinking hydrosilylation, in which the compositions contain not only (η-diolefin)(σ-aryl)platinum complexes but also a free-radical photoinitiator which absorbs actinic radiation and in this way contributes to an increase in the light yield. This combination of (η-diolefin)(σ-aryl)platinum complex and free-radical photoinitiator makes initiation of a hydrosilylation reaction with an accelerated crosslinking process possible. However, the use of an additional component must in principle be considered to be disadvantageous since it makes the production process correspondingly more complicated.
On the other hand, EP 0 398 701 B1 claims Pt(II)-β-diketonate complexes which have the advantage of a long pot life combined with a short gel time on illumination. However, the relatively polar compounds have the disadvantage of poor solubility in the silicone matrix and therefore have only limited suitability for many applications.
EP 0 146 307 B1 discloses (η5-cyclopentadienyl)tri(σ-alkyl) platinum(IV) complexes which display good solubility in the silicone matrix. More highly concentrated solutions can also be achieved using the complexes. However, a considerable disadvantage of the compounds is their relatively high vapor pressure and their volatility. As a result, it is not possible to rule out a change in the platinum concentration when vacuum is applied in the production or processing of the silicone elastomer. A further consequence is that contamination of the air of the room with toxicologically problematic platinum compounds cannot be ruled out.
In EP 0 358 452 B1, sensitizers are added to the compositions containing (η5-cyclopentadienyl)tri(σ-alkyl) platinum(IV) complexes as catalyst in order to shift the wavelength of the incoming light required for crosslinking to longer wavelengths. The advantage resulting therefrom is that the mixtures can be cured using visible light instead of ultraviolet light.
EP 0 561 893 B1 describes radiation-crosslinkable compositions containing (η5-cyclopentadienyl)tri(σ-alkyl)platinum(IV) complexes together with a free-radical photoinitiator which absorbs actinic radiation and in this way contributes to an increase in the light yield. This combination makes an increase in the quantum yield for the mixtures used possible. The resulting increase in the dark reactivity has to be regarded as a disadvantage. In addition, the use of an additional component makes the production process more expensive and also increases the materials costs.
EP 1 803 728 A1 discloses modified (η5-cyclopentadienyl)tri (σ-alkyl)platinum(IV) complexes which bear specific substituents (naphthyl, anthracenyl, etc.) on the cyclopentadienyl ring in order to increase the quantum yield and to shift the light wavelength required for activation to longer wavelengths. However, the attachment of aromatic rings has an adverse effect on the solubility of the complexes in the silicone matrix. These compounds, too, have the disadvantage of volatility.
Many of the platinum complexes described are also used in CVD (chemical vapor deposition) applications, which in itself indicates a high volatility of the compounds described. In the technical literature, the vapor pressure of the complexes CpPtMe3 and MeCpPtMe3 was determined experimentally by Z. Xue et al., (J. Am. Chem. Soc., 1989, 111, 8779). The high volatilities measured represent a not inconsiderable risk from many points of view, especially because of the danger of platinum contamination of the working area. In addition, inhalation of the platinum compounds can also represent a health risk. In addition, the ability of the relatively small, nonpolar molecules to pass through the skin, which is attributable to the chemical structure, has to be considered to be a further not inconsiderable disadvantage of the platinum complexes described.
In summary, it can be said that none of the silicone compositions known hitherto which can be crosslinked by means of visible and/or UV radiation satisfactorily fulfill the requirements which have to be met by such silicone compositions which are employed, in particular, for production in an industrial environment.